Microporous silica membranes deposited on porous supports by filtration

Water based particulate silica sols have been coated onto Anodisc® filters by filtration. The membranes prepared by this technique are more uniform than those formed by slip-casting. The average diameter of the silica particles used in these studies is 6 nm. Unsupported silica membranes formed from these sols have a microporous structure. The adhesion between silica and the alumina support is influenced by the sol pH. Coating thickness can be controlled by the concentration and volume of the sol filtered. Polyvinyl alcohol (PVA) was used to improve adhesion and to prevent cracking during drying. When the PVA/SiO₂ ratio by weight is less than 20%, the membranes retain their microporosity after firing. The membranes prepared by this filtration method have their pore size in Knudsen diffusion range.     

Production of Titanate Microspheres by Sol-Gel and Spray-Drying

Porous titanate precursor microspheres (20–60 m in diameter), with a high sorption capacity for radioactive wastes from nuclear reprocessing plants, have been produced on a 50 kg scale by spray-drying precursor sols. Well-dispersed, stable sols were produced by hydrolyzing acetic acid modified tetraisopropyltitanate and peptizing the titania hydrolysate with acidic zirconia sol. The resulting TiO2 /ZrO2 sols were routinely concentrated to 900 g dm–3 (oxide basis) and exhibited excellent stability. These sols were subsequently mixed with dispersible alumina powder and partially aggregated by adding calcium and barium nitrate salts. The resulting sols were spray-dried to produce microspheres with controlled porosity and morphology. The properties of the spray-dried powder were very dependent upon the chemical properties of the precursor sol. In particular, hollow spheres were produced from well-dispersed sols, whereas solid spheres could be produced from partially-aggregated sols.     

Surface electric and titration behaviour of fumed oxides

Adsorption of Pb(II), Sr(II), and Cs(I) on fumed silica, alumina, titania, silica/titania (ST), silica/alumina (SA), and alumina/silica/titania (AST) reveals that mixed oxides containing titania have a greater adsorptive capability in respect to metal cations than individual and SA oxides. Pyrocarbon deposits on fumed oxides enhance the adsorption of metal ions. Calculations of electrophoretic potential (ζ) with consideration for the porosity of aggregates of primary particles of AST show a significant influence of surface alumina (at pH<8) and titania and silica (at pH>8) on the ζ values. The effective diameter of particles (D_ef) of fumed oxides in aqueous media depends on pH for AST stronger than for ST (between isoelectric points (IEPs) of titania and alumina). A significant difference in the pH values of IEP and point of zero charge is observed for AST samples. A pyrocarbon influence on the ζ potential depends on the type of oxide matrix, since ζ increases for certain samples but for others it decreases. These changes depend nonlinearly on pH as well as the secondary particle size distributions (SPSDs) and D_ef.     

The complexes of macromolecules and metal nanoparticles: pseudo-template synthesis and behavior

Metal sols composed of metal nanoparticles (1 - 10 nm in diameter) protected with polymer molecules may be regarded as dispersions of polymer-metal complexes formed due to cooperative non-covalent (e.g., hydrophobic, coulombic) interaction of polymer chains with the surface of metal nanoparticles. The sols are commonly prepared by reducing of metal ions in solutions of appropriate polymers. The interactions between macromolecules and nanoparticles are reversible. In the case of long polymer chains and minute particles, the equilibrium constant of the reaction exponentially depends on the surface area of the particle. The probability of mutual “recognition” (complex formation) of growing particle and a macromolecule rapidly increases from practically zero to practically unity in narrow interval of the particle's diameters. The recognition is followed with the shadowing of the particles and the stop of their growths. Such kind of processes was termed “pseudo-template”. In frame of the concept of pseudo-template processes can be estimated: (1) the conditions at which sol particles of desirable size can be prepared, (2) the influence of temperature, polymer concentration, nanoparticles size, and other conditions on the stability of polymer - particle complex having been prepared, and (3) the conditions at which stable sol does not exist and can not be prepared at all. The interactions between metal nanoparticles and macromolecules are highly selective regard to the structure of polymer chains. The property can be effectively used for the control the size characteristics of metal nanoparticles (in course of their formation) and the stability of metal sols. The selectivity provides high conversions in catalytic chemical modification reactions in which a macromolecule is the substrate and a component of the catalyst in the same time. As an example, the hydrolysis of lactame groups in monomer unites of poly(N-vinyl pyrrolidone) catalyzed with copper sols is discussed.     

A Comparison of the Nanostructure of Lead Zirconate,Lead Titanate and Lead Zirconate Titanate Sols

Sol-gel processing has been widely used for the fabrication of lead zirconate titanate (PZT) thin films. To successfully and consistently make high quality thin films for different applications, we must develop a fundamental understanding of the structures of the sols. In this study, the characters of lead titanate (PT) and lead zirconate (PZ)sols were studied by measuring the rheological properties and particle sizes in them and comparing their behaviours. The average particle sizes in unhydrolysed PT, PZ and PZT sols are 11.5, 1.0, and 6.0 nm, respectively. PT sol has the highest rate of hydrolysis. It gels at about 24 h after hydrolysis. PZ and PZT sols have a quite similar feature in hydrolysis. The reasons for the differences in the hydrolysis behaviour of the different types of sol are discussed in terms of a model which indicates that the inhomogeneous sols consist of 5 to 6 nm PT particles surrounded by much smaller PZ particles, which tend to dominate the sol behaviour.     

Olefin polymerization using supported metallocene catalysts: development of high activity catalysts for use in slurry and gas phase ethylene polymerizations

Reaction of hydroxylated silica and alumina supports with methyl aluminoxane in toluene suspension provides chemically modified supports suitable for use in slurry and gas-phase polymerizations of ethylene or propylene on treatment with a variety of metallocene dichloride complexes. In particular, aluminas derived from calcination of sol–gel precursors feature high degrees of surface hydroxylation in comparison with commercially available silica (or even alumina) of similar surface area and total porosity. This feature provides a mechanism for increasing the amount of aluminoxane on the former supports, such that commercially acceptable productivities (>10 kg PE/g support×h) are observed at relatively low, total levels of aluminoxane or other alkylaluminum compounds in slurry or gas-phase polymerizations, respectively. A variety of evidence indicates that leaching of active catalyst from these alumina supports occurs to a minor extent under slurry conditions, particularly at higher temperatures in the presence of additional aluminoxane. At lower temperatures, this does not occur to an appreciable extent but the morphology and bulk density of the polymer formed is unsuitable for use in a gas-phase process. This can be attributed to the method for synthesis of the sol–gel alumina precursor which results in irregular particles with a broad particle size distribution. Copolymerization of ethylene with 1-octene or 1-hexene results in formation of linear, low density, PE with a narrow composition distribution as revealed by temperature rising elution fractionation. These studies indicate that less comonomer is incorporated using these supported metallocene catalysts than their soluble analogues under otherwise identical conditions. Finally, some of the resins prepared under slurry conditions (and to a lesser extent in a gas-phase process), exhibit properties consistent with the presence of low levels of long-chain branching; this feature appears to be reasonably general for a variety of simple metallocene complexes.     

Preparation and characteristics of spinnable sol and continuous mullite fibers with nano silica addition

The spinning precursor sols for the continuous mullite-based fibers were prepared by adding nano-silica to substitute part of silica sol. The effect of SiO₂ nanoparticles on the particle evolution models, polymerization degree and solid content of the sol,and the spinning length and sintering behavior of the fibers was investigated. The results were shown that the addition of nano silica enhanced the polymerization degree and extended the spinnable range of the sol. The appropriate polymerization degree (B value) for this sol system was 1.885–2.145. The grain diameter decreased from 39.6 to 25.9 nm with increasing the nano-silica content to 20 %, and then, it increased to 41.2 nm with increasing the nano-silica content to 100 %. The appropriate content of nano-silica powders would reduce the grain diameter. However, it had no influence on the linear growth model, homogeneity and solid content of the precursor sol.     

Ion exchange in carboxylated latices — AUC studies using sedimentation and density gradient techniques

Crosslinked highly carboxylated acrylic latices with narrow particle size distributions were prepared by emulsion polymerization and characterized carefully by different AUC techniques (particle size distributions and particle density measurements). The acid form of those latices was neutralized with metal oxides like MgO, CaO, ZnO, or PbO in order to obtain the corresponding salt form of the latices which again were characterized carefully. The kinetics of the ion exchange between latex particles were studied by mixing, for example, the acid and the salt form of the latices monitoring the density distribution of the latex particles by density gradient ultracentrifugation. With all latices the hydrogen-metal ion exchange tends to be a complete one provided this process is given a sufficiently long exchange time. Theoretical models are provided which yield a qualitative explanation of the experimental data.     

Size Distribution Evolution of NiO x H y and Au: NiO x H y Sols

The present work reports on the preparation of nickel oxi/hydroxide and gold composite nanoparticulate sols. The precursor was prepared as an alcoholic suspension of nickel oxi/hydroxide. Controlled amounts of 2,4-pentanedione (AcAc) were added to the nickel salt suspension and lead to the formation of a nickel complex. The dependence of the particle size with AcAc titration was followed by Small Angle X-Ray Scattering (SAXS). The results showed that the particle size distribution shifts towards low values with the increase in the AcAc concentration. The complex, obtained by adding an excess of the AcAc ligand, [Ni(AcAc)₂]·2H₂O, was identified by FTIR spectroscopy and X-ray diffraction. An AcAc specific volume was chosen in order to obtain a homogeneous sol of nickel oxi/hydroxide to which gold was added. The size distribution of the Au particles in this sol was followed by SAXS.     

Mechanism of pore formation and structural characterization for mesoporous Mg-Al composite oxides

Mg-AI layered double hydroxides (LDH) with different particle sizes were prepared using different aging times at high supersaturation by a new method developed in our laboratory. The key features of this method are a very rapid mixing and nucleation process followed by a separate aging process. By calcination of LDH at 500癈, mesoporous Mg-AI composite oxides with an extremely narrow pore size distribution were produced. The crystal structure of the Mg-AI composite oxides was a multiphasic one consisting of MgO-like crystals and a layered material.     

Ordered mesoporous alumina-supported metal oxides

The one-pot synthesis of alumina-supported metal oxides via self-assembly of a metal precursor and aluminum isopropoxide in the presence of triblock copolymer (as a structure directing agent) is described in detail for nickel oxide. The resulting mesoporous mixed metal oxides possess p6 mm hexagonal symmetry, well-developed mesoporosity, relatively high BET surface area, large pore widths, and crystalline pore walls. In comparison to pure alumina, nickel aluminum oxide samples exhibited larger mesopores and improved thermal stability. Also, long-range ordering of the aforementioned samples was observed for nickel molar percentages as high as 20%. The generality of the recipe used for the synthesis of mesoporous nickel aluminum oxide was demonstrated by preparation of other alumina-supported metal oxides such as MgO, CaO, TiO 2, and Cr 2O 3. This method represents an important step toward the facile and reproducible synthesis of ordered mesoporous alumina-supported materials for various applications where large and accessible pores with high loading of catalytically active metal oxides are needed.     

镁铝复合氧化物载体的制备与性质研究

以硝酸镁和硝酸铝为原料，用氨水溶液作为pH调节剂，采用共沉淀法制备了镁铝复合氧化物载体，研究了制备过程中镁铝比、pH调节剂种类、水解过程pH值的大小、反应温度、焙烧温度及回流晶化温度对复合氧化物载体理化性质的影响。并以RFCC汽油加氢脱硫为探针反应，考察了以镁铝复合氧化物为载体的催化剂选择性加氢脱硫性能。实验结果表明，在镁铝分子比为10、反应温度为80℃、pH值为9.5条件下制备的镁铝复合氧化物载体具有适宜的比表面积和均匀的孔分布，且晶型较完整，结晶度高。以该复合氧化物为载体制备的催化剂具有良好的RFCC汽油选择性加氢脱硫反应性能。     

Studies of Lead Zirconate Titanate Sol Ageing Part I: Factors Affecting Particle Growth

The formation and growth of polymeric particles during the hydrolysis and condensation of PbZr_0.3Ti_0.7O₃ (PZT 30/70) precursor solutions have been studied as functions of time by using photon correlation spectroscopy (PCS). Particle shape was deduced by measuring the rheological properties of the sols and was found to remain chain-like during sol ageing. Various factors that affect the ageing of PZT sols have been investigated. Ethylene glycol (EG) was found to greatly accelerate the hydrolysis and condensation of the acid-modified Pb-free Zr/Ti solutions but had little effect on the particle growth for the Pb-containing PZT sols. Temperature and H₂O concentration were found to have major effects on the ageing behaviors. The higher the temperature and H₂O concentration, the higher the particle growth rates and the faster the sol gels.     

Bismuth titanate thin films prepared by wet-chemical techniques: effect of sol ageing time

In this work, layered perovskite bismuth titanate (Bi₄Ti₃O₁₂) thin films were fabricated on α-alumina substrates by spin coating process. Precursor sol was prepared by sol–gel process from bismuth nitrate and titanium butoxide in concentrated acetic acid, with diethylamine as a stabilizer. Processes occurring in the precursor sol were followed in the ageing period of 20 days. Thin films prepared from the as-synthesized and aged sols are crack-free, with the thickness of ~1 μm, uniform surface texture and rounded grains having grain size in nanometer range. Sintering of thin films was performed at various temperatures, and sintered thin films exhibited dense structure, fully crystallized with typical Aurivillius phase and without any preferred orientation and impurity phase. The influence of ageing of the precursor sol on the microstructure of obtained thin films was also investigated. Direct relation between hydrodynamic diameter of precursor particles and the morphology and the grain size of the obtained films was observed.     

PVP改性氧化硅溶胶的微结构和流变性

以正硅酸乙酯为前驱体,聚乙烯吡咯烷酮（PVP）为聚合物改性添加剂,在碱性条件下制备氧化硅溶胶,通过小角X射线散射测量了改性溶胶的微结构参数,辅以动态光散射观测溶胶颗粒生长,研究了聚合物量、碱催化剂量和水量对溶胶颗粒度、分形特征以及流变性的影响规律.由于PVP链中强极性的分子内酰胺基团和氧化硅颗粒表面的羟基形成氢键,致使溶胶颗粒被聚合物链包裹,严重阻碍了溶胶颗粒的生长,使凝胶时间延长,流变性发生变化,同时对溶胶颗粒的微结构产生影响.     

Polystyrene-silica nanocomposite particles via alcoholic dispersion polymerization using a cationic azo initiator

Submicrometer-sized polystyrene-silica nanocomposite particles have been prepared by alcoholic dispersion polymerization of styrene using commercial alcoholic silica sols of 13 or 22 nm diameter as the sole stabilizing agent. The key to the formation of colloidally stable nanocomposite particles is the selection of a cationic azo initiator (use of nonionic or anionic initiators leads either to the formation of silica-stabilized polystyrene latex particles with very low silica contents or to the precipitation of polystyrene, respectively). Neither surface modification of the silica sol nor the addition of surfactant or polymeric stabilizers is required for successful nanocomposite syntheses. The purified polystyrene-silica nanocomposite particles have relatively narrow particle size distributions, with mean diameters ranging from 331 to 464 nm as judged by disk centrifuge photosedimentometry. Thermogravimetric analyses indicated mean silica contents of 13-26 wt. %, depending on the synthesis conditions. Calcination of the polystyrene-silica nanocomposite particles leads to the formation of hollow silica shells, which indicates a well-defined core-shell morphology for the original nanocomposite particles.     

Dispersion and aggregation of nanoparticles derived from colloidal droplets under low-pressure conditions

Formation of individually dispersed nanoparticles or compactly aggregated nanoparticles from sols via a spray-drying route at low pressure was investigated experimentally. Silica sol was used as a sample material. Effects of operating temperature, colloid size, sol concentration, pressure, pH and zeta potential of sols on the morphology of product particles were investigated. From the experimental results, it was shown that dispersed nanoparticles could be obtained at a relatively low pressure (20 Torr) and low temperature (200 degrees C). The experiment also showed that dispersed nanoparticles could be achieved by careful control of the interfacial energy (pH value) of the colloidal precursor. A possible mechanism of sol-to-dry-particle formation in the spray-drying process at low pressure is suggested, based on the experimental results and the available theories. This mechanism was able to explain the experimental results well.     

Surface structure and properties of mixed fumed oxides

A variety of fumed oxides such as silica, alumina, titania, silica/alumina (SA), silica/titania (ST), and alumina/silica/titania (AST) were characterized. These oxides have different specific surface areas and different primary particle composition in the bulk and at the surface. These materials were studied by FTIR, NMR, Auger electron spectroscopy, one-pass temperature-programmed desorption with mass spectrometry control (OP TPDMS), microcalorimetry, and nitrogen adsorption. Nonlinear changes in the surface content of alumina in SA and AST and titania in ST and AST samples with increasing oxide content along with simultaneous changes in their specific surface area cause complex dependencies of the heat of immersion in water and desorption of water on heating on the structural parameters. Simultaneous analysis of changes in the surface phase composition, in the concentration of hydroxyls, and in the structural characteristics reveals that at a low content of the second phase the structural characteristics (e.g., S(BET)) are predominant; however, at a large content of these oxides the phase composition plays a more important role.     

Preparation of Alumina Aerogel Films by Low Temperature CO2 Supercritical Drying Process

Alumina aerogel thin films were formed by a new synthesis route. Sols were prepared by the Yoldas process. Gels were formed by sol evaporation in a few hours. Films were prepared by dip coating glass or alumina substrates into both the sols and the gels. Aerogel films with special morphology were produced for the first time by exchanging the film solvent with acetone after the dip coating, followed by supercritical drying. The morphology of the films, studied by SEM and TEM, consists of fiber-like network of round chains (≈0.1 μm thick), and pores (0.1–0.5 μm in diameter). It is shown that the fibers contain a homogeneous arrangement of sol particles, 2–4 nm in size. Formation of this microstructure can be attributed to phase separation in the alumina-water-acetone system in a 2D film geometry. A conceptual model for the film development is proposed.     

金属氧化物纳米催化的形貌效应

纳米结构催化剂的设计与制备是多相催化的核心问题之一．提高催化活性的传统方法是减小催化剂粒子的尺寸以暴露更多的表面活性位,即纳米催化中的尺寸效应,但这种方法往往带有一定程度的经验性和随机性．近年来,随着纳米材料科学的快速发展,在溶液体系中通过自下而上的合成技术已经可以在纳米尺度上有效调变固体催化剂粒子的形貌．通过纳米催化材料的形貌可控合成,可选择性地暴露高活性或特定能量晶面,从而大幅度提升催化反应活性、选择性和稳定性,也就是纳米催化中的形貌效应,这也是当前纳米催化研究的热点之一．本文以作者近年来研究的C0304、CeO2和Fe2O3为重点,总结了纳米结构金属氧化物在多相催化反应中的形貌效应,分析了氧化物暴露品面的化学性质对催化性能的作用机制．这种基于形貌效应的纳米催化不仅加深了在纳米尺度甚至原子层次上对催化剂构一效关系的认知,而且对设计和开发实用高效催化剂也具有重要的理论价值．